VMware Software-Defined Storage. Martin Hosken

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СКАЧАТЬ devices in a new way, and transforms storage management by enabling full virtual machine awareness from the storage array. Based on a T10 industry standard, Virtual Volumes provides a unique level of integration between vSphere and third-party vendors’ storage hardware, which significantly improves the efficiency and manageability of virtual workloads.

      Virtual Volumes virtualizes shared SAN and NAS storage devices, which are then presented to vSphere hosts, providing logical pools of raw disk capacity, called a virtual datastore. Then, Virtual Volume objects, which represent virtual disks and other virtual machine entities, natively reside on the underlining storage, making the object, or virtual disk, the primary unit of data management at the array level, instead of a LUN. As a result, it becomes possible to execute storage operations with virtual-machine, or even virtual-disk, granularity on the underlining storage system, and therefore provide native array-based data services, such as snapshots or replication, to individual virtual machines.

      To facilitate a simplified and unified approach to management, all this is done with a common storage-policy-driven mechanism, which encompasses both Virtual SAN storage resources and Virtual Volumes external storage, into a single management plane. Virtual Volumes is covered in more detail in Chapter 8, “Policy-Driven Storage Design with Virtual Volumes.”

      Classic and Next-Generation Storage Models

      This book refers to storage technologies as either classic or next-generation. Because these terms can have multiple meanings, this section provides an overview of each to clarify.

      This book uses classic storage model to describe the traditional shared storage model used by vSphere. This typically includes LUNs, VMFS-based volumes and datastores, or NFS mount points, with a shared storage protocol providing I/O connectivity. Despite its constraints, this model has been successfully employed for years, and will continue to be used for some time by IT organizations and cloud service providers across the industry.

      The next-generation storage model refers to VMware’s software-defined solutions, Virtual SAN and Virtual Volumes, which bring about a new era in storage design, implementation, and management.

      As addressed earlier in this chapter, the primary aim of VMware’s software-defined storage model is to bring about simplicity, efficiency, and cost savings to storage resources. The model does this by abstracting the underlining storage in order to make the application the fundamental unit of management across a heterogeneous storage platform. With both Virtual SAN and Virtual Volumes, VMware moves away from the rigid constraints of the classic LUNs and volumes, and provides a new way to manage storage on a per virtual machine basis, through its more flexible policy-driven approach.

      However, before addressing these next-generation storage technologies, you first need to understand the approach taken to storage over the last generation of vSphere-based virtualization platforms, and see how the VMware stack itself interacts with storage resources to provide a flexible, modern virtual data center.

      This first chapter has addressed the VMware storage landscape, processes associated with storage design, and challenges faced by vSphere storage administration teams when maintaining complex, heterogeneous storage platforms on a daily basis for enterprise IT organizations and cloud service providers. The next chapter presents many of the essential design considerations based on the classic storage model previously outlined.

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Chapter 2

      Classic Storage Models and Constructs

      This chapter covers the design considerations for deploying classic storage technologies in a VMware-based virtual data center, and addresses the primary storage concepts that impact the platform design of the storage layer.

      Classic Storage Concepts

      Storage infrastructure is made up of a multitude of complex components and technologies, all of which need to interact seamlessly to provide high performance, continuous availability, and low latency across the environment. For students of vSphere storage, understanding the design and implementation complexities of mixed, multiplatform, multivendor enterprise or service provider–based storage can at first be overwhelming. Gaining the required understanding of all the components, technologies, and vendor-specific proprietary hardware takes time.

      This chapter addresses each of these storage components and technologies, and their interactions in the classic storage environment. Upcoming chapters then move on to next-generation VMware storage solutions and the software-defined storage model.

      This classic storage model employs intelligent but highly proprietary storage systems to group disks together and then partition and present those physical disks as discrete logical units. Because of the proprietary nature of these storage systems, my intention here is not to address the specific configuration of, for instance, HP, IBM, or EMC storage, but to demonstrate how the vSphere platform can use these types of classic storage devices.

In the classic storage model, the logical units, or storage devices, are assigned a logical unit number (LUN) before being presented to vSphere host clusters as physical storage devices. These LUNs are backed by a back-end physical disk array on the storage system, which is typically served by RAID (redundant array of independent disks) technology; depending on the hardware type, this technology can be applied at either the physical or logical disk layer, as shown in Figure 2.1.

Figure 2.1 Classic storage model

      The LUN, or storage device, is a virtual representation of a portion of physical disk space within the storage array. The LUN aggregates a portion of disk space across the physical disks that make up the back-end system. However, as illustrated in the previous figure, the data is not written to a single physical device, but is instead spread across the drives. It is this mechanism that allows storage systems to provide fault tolerance and performance improvements over writing to a single physical disk.

      This classic storage model has several limitations. To start with, all virtual disks (VMDKs) on a single LUN are treated the same, regardless of the LUN’s capabilities. For instance, you cannot replicate just a single virtual disk at the storage level; it is the whole LUN or nothing. Also, even though vSphere now supports LUNs of up to 64 terabytes, LUNs are still restricted in size, and you cannot attach more than 256 LUNs to a vSphere host or cluster.

      In addition, with this classic storage approach, when a SCSI LUN is presented to the vSphere host or cluster, the underlying storage system has no knowledge of the hypervisor, filesystem, guest operating system, or application. It is left to the hypervisor and vCenter, or other management tools, to map objects and files (such as VMDKs) to the corresponding extents, pages, and logical block address (LBA) understood by the storage system. In the case of a NAS-based NFS solution, there is also a layer of abstraction placed over the underlying block storage to handle file management and the associated file-to-LBA mapping activity.

      Other classic storage architecture challenges include the following:

      • Proprietary technologies and not commodity hardware

      • Low utilization of raw storage resources

      • Frequent overprovisioning of storage resources

      • Static, nonflexible classes of service

      • Rigid provisioning methodologies

      • Lack of granular control, at the virtual disk level

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